Self-healing materials have received considerable attention due to their great potential to diminish degradation and reduce the maintenance cost. Since the first generation self-healing material based on the ring opening metathesis polymerization (ROMP) of encapsulated dicyclopentadiene (DCPD) in the presence of Grubbs' catalyst particles (S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N. Brown and S. Viswanathan, Nature, 2001, 409, 794-797) microencapsulation has been one of the most efficient and widely used approaches in self-healing materials development. Poly(urea-formaldehyde) (PUF) microcapsules containing DCPD as healing agent were prepared through an in situ polymerization in oil-in-water emulsion (E. Brown, M. Kessler, N. Sottos and S. White, J. Microencapsulation, 2003, 20, 719-730; M. R. Kessler, N. R. Sottos and S. R. White, Composites: Part A, 2003, 34, 743-753) and the capsules size was further reduced to nanometer scale with the assistance of a sonication technique (B. J. Blaiszik, N. R. Sottos and S. R. White, Compos. Sci. Technol., 2008, 68, 978-986). Linseed oil (C. Suryanarayana, K. C. Rao and D. Kumar, Prog. Org. Coat., 2008, 63, 72-78), amines (D. A. McIlroy, B. J. Blaiszik, M. M. Caruso, S. R. White, J. S. Moore and N. R. Sottos, Macromolecules, 2010, 43, 1855-1859) and epoxy resins (L. Yuan, G. Liang, J. Xie, L. Li and J. Guo, Polymer, 2006, 47, 5338-5349) were also microencapsulated for self-healing applications. To avoid the contamination of catalyst by the host matrix, a dual capsule system was reported (S. Cho, H. Andersson, S. White, N. Sottos and P. Braun, Adv. Mater., 2006, 18, 997-1000; S. H. Cho, S. R. White and P. V. Braun, Adv. Mater., 2009, 21, 645-649) and this approach has shown good self-healing and corrosion protection features. Most of the capsules applied for self-healing purpose so far were made from PUF, polyurethane (PU) and polyurea. To overcome this limitation, a double-walled polyurethane-poly(urea formaldehyde) (PU-PUF) microcapsule was recently developed through the combination of interfacial polymerization of PU and in situ polymerization of PUF in a single batch reaction (M. M. Caruso, B. J. Blaiszik, H. Jin, S. R. Schelkopf, D. S. Stradley, N. R. Sottos, S. R. White and J. S. Moore, ACS Appl. Mater. Interfaces, 2010, 2, 1195-1199.) Other approaches such as hollow glass fiber embedment, microvascular system, and electrospun hollow fibers have also been extensively investigated for self-healing materials development, and more recently there was reported an oxetane-substituted chitosan precursor incorporated PU showing good scratch closure performance within half an hour under sunlight (B. Ghosh and M. Urban, Science, 2009, 323, 1458-1460).
Isocyanates are reactive with moisture, and can be used as a potential healing agent to develop one-part, catalyst-free self-healing materials that are exposed to moist or aqueous environments. On the other hand, however, the high reactivity of isocyanates brings the difficulty for processing. Previous research on encapsulation of isocyanate has been mainly restricted to its blocked form or solid state (I. W. Cheong and J. H. Kim, Chem. Commun., 2004, 2484-2485; H. Yang, S. Mendon and J. Rawlins, eXPRESS Polym. Lett., 2008, 2, 349-356.
Yang et al. for the first time reported in Macromolecules, 2008, 41, 9650-9655, the microencapsulation of liquid isocyanate monomer. Less reactive isophorone diisocyanate (IPDI) was encapsulated by polyurethane microcapsules based on the polymerization of toluene diisocyanate (TDI) prepolymer that was cautiously in-house synthesized. However, there is still a need to provide further microcapsules that are suitable for the microencapsulation of liquid isocyanate monomer for instant and effective corrosion protection of coating upon damage event or for extended service life of corrosion protection of undamaged coating. If there is damage, such microcapsules serve as healing source to seal the damage in the coating. If there is no damage, the microcapsules can trap and react with slowly diffused water/moisture in the coating to extend the service life thereof.